This invention relates to a central system fluid composition.
A central vehicle hydraulic system, or simply a central system, denotes a system whereby hydraulic devices are mounted on a motor vehicle to actuate a power steering device, a brake, an automatic transmission device, a hydraulic suspension device, a windshield wiper, a seat actuator, a wind actuator, etc. through a single hydraulic source using a single kind of oil. A fluid used in this system is called a central system fluid.
The properties required of the central system fluid are regulated in U.S.A. by SAE 71R1 Standards (for mineral oils) and SAE 71R2 Standards (for synthetic oils). These standards were set down on the basis of the properties required of both a power steering fluid and a brake fluid. These standards are diverse and severe and specifically include the following.
1. They should have excellent flowability at low temperatures.
2. They should have excellent shear stability.
3. They should be operable over a wide temperature range.
4. They should have high boiling and flash points.
5. They should not form precipitates or condensates at low temperatures.
6. They should have a reduced tendency to bubbling.
7. They should have excellent lubricity and oxidation stability.
8. They should not corrode the metallic portions of a hydraulic system nor excessively swell the rubber portions thereof.
The present inventors already suggested a fluid which is acceptable by the SAE 71R1 Standards, and disclosed it in U.S. Pat. No. 4,031,020 as a hydraulic composition consisting essentially of at least one hydrocarbon base oil selected from the group consisting of (A) polybutene, (B) a homopolymer or copolymer of an alphaolefin having 2 to 12 carbon atoms and (C) a nuclearly hydrogenated product of an aromatic hydrocarbon, and at least one viscosity index improver selected from (1) polymethacrylates and (2) polyolefins.
In recent years, the central system fluid has also been required to have excellent performance at high temperatures, such as high flash and initial boiling points and a reduced loss of weight by evaporation, from the standpoint of safety and in view of a temperature rise within the engine room resulting from measures taken to control motor vehicle exhausts. Moreover, the compatibility of the central system fluid with rubber material used in the central hydraulic system has been regarded as more important than before, and in addition to the aforesaid SAE Standards, the central system fluid is also required to have an aniline point within a specified desirable range. The properties of the central system fluid which have been newly required are tabulated
______________________________________ Newly required SAE 71R1 SAE 71R2 properties Standards Standards ______________________________________ 1. Viscosity cSt (100.degree. C.) above -- -- 6.3 2. Low temperature below below below viscosity cSt 2000 2000 1800 (-40.degree. C.) 3. Flash point (.degree.C.) above above above 140 107.2 96.1 4. Initial boiling point above above above (.degree.C.) 240 204.4 204.4 5. Aniline point (.degree.C.) 80-100 -- -- 6. Weight loss by below -- -- evaporation 15% by (100.degree. C. .times. 48 hrs) weight ______________________________________
The aforesaid SAE Standards themselves are very severe, and to the best of the knowledge of the present Applicants, except the hydraulic composition disclosed in the above-cited U.S. Pat. No. 4,031,020, there has been scarcely any fluid composition which fully meets the SAE standards. It has been considered extremely difficult therefore to provide a fluid which meets the above tabulated various properties in addition to the SAE Standards.